Corneal powers measured with a rotating Scheimpflug camera

Background/aim Keratometry measures the anterior corneal curvature only. Corneal power is calculated by multiplication with the keratometric refractive index, which takes into account the average negative posterior corneal power. The aim of this study was to calculate and compare various expressions for total corneal power assessed with Scheimpflug camera techniques, which also measure the posterior corneal curvature. Methods We used the Pentacam rotating Scheimpflug camera to measure the equivalent power, total corneal refractive power (based on Snell's law ray tracing), and simulated keratometry (keratometric refractive index=1.3375) over the central 3.0mm zone in 951 eyes. The keratometric refractive index of the equivalent power and the total corneal refractive power was calculated as the ratio between these values and the anterior corneal curvature. Results The equivalent power, total corneal refractive power, and simulated keratometry all differed statistically significantly (analysis of variance, p<0.001) and averaged 42.26 (1.46) dioptres (D), 42.78 (+/- 1.51) D and 43.42 (+/- 1.49) D. The calculated keratometric refractive indices for equivalent and total corneal refractive power averaged 1.3284 (+/- 0.0009) and 1.3324 (+/- 0.0015). The error of using these calculated keratometric refractive indices rather than the measured values for equivalent and total corneal refractive power averaged 0 (+/- 0.11D) and -0.01D (+/- 0.19). Conclusions Pentacam rotating camera assessment of total corneal power over the central 3.0mm zone differed significantly for simulated keratometry, equivalent power and Snell's law ray tracing.